Third generation partnership project (3GPP) and Long Term Evolution (LTE) mobile telecommunication systems provide high data rate, lower latency and improved system performances. With the rapid development of “Internet of Things” (IOT) and other new user equipment (UE), the demand for supporting massive machine communications increases exponentially. The probability that multiple devices choose the same preamble to perform the contention based random access channel (RACH) is higher than the current human type communications. If the preamble is detected in the eNB side, eNB will response an RAR (Random Access Response) with RAPID (Random Access Preamble ID) by using RA-RNTI indicating the UL grant. The UEs choosing the same RAPID will transmit their Message 3 (Msg3) in the same scheduled UL grant. The multiple devices will transmit Msg3 in the indicated UL grant in the RAR by using the same resource and the same demodulation reference signal (DMRS). When the eNB performs the channel estimation, it will see a composite channel from all transmitted UE. If the eNB applies the composite channel result to decode the UE specific Msg3 in the physical uplink shared channel (PUSCH), with high probability, the PUSCH cannot be decoded correctly.
When the preamble is detected by eNB, eNB will indicate a timing advance in RAR to a specific UE. All the UEs which are transmitted the same preamble ID and receive the same RAR will apply this timing advance (TA) to transmit their Msg3. Although the location of the UEs may be different, as long as the receiving of all Msg3 (TA and delay spread) is within cyclic prefix length at the eNB, the eNB will treat these signals the same as multipath phenomena. Since these Msg3 messages are carrying different contents, they will severely interfere with each other, such as co-channel interference.
If eNB cannot successfully decode Msg3, it will transmit NACK to the UEs. The UEs receive the NACK will re-transmit their redundancy version of Msg3s. In such case, with high probability, the transmission and re-transmission of Msg3 may collide until the maximum number of Msg3 transmission is reached. The UE waits for the expiration of the mac-ContentionResolutionTimer before it retries the preamble again. It is estimated that with the increasing number of UEs/devices in the wireless system, such as supporting machine type communications, the probably of Msg3 collision will be high.
Improvements and enhancements are needed for LTE RACH procedure enhancement.